1. Technical Field
The present invention relates to a treating agent for alignment of liquid crystal, and more particularly, it relates to a novel treating agent for liquid crystal alignment to be used for a method in which liquid crystal molecules are aligned by irradiating ultraviolet rays, etc. on the surface of a polymer thin film without rubbing treatment.
2. Background Art
A liquid crystal display device is a display device utilizing an electro optical change of liquid crystal, and as an apparatus, it is small in size and light in weight and has an attractive feature such that the power consumption is small. Accordingly, in recent years, it has undergone remarkable developments as a display device for various types of displays. Among them, an electric field effect type liquid crystal display device of twisted nematic type (TN type) is a typical one, wherein nematic liquid crystal having positive dielectric anisotropy is employed, liquid crystal molecules are aligned at the respective interfaces of a pair of electrode substrates disposed to face each other, in parallel with the substrates, and the two substrates are combined so that the alignment directions of liquid crystal molecules will cross each other.
In such a TN type liquid crystal display device, it is important that long axes directions of liquid crystal molecules are uniformly aligned in parallel on the substrate surface and further that liquid crystal molecules are aligned with a certain inclined alignment angle (hereinafter referred to as a tilt angle) to the substrate.
Heretofore, two methods have been known as typical methods for aligning liquid crystal molecules in such a manner. The first method is a method wherein an inorganic substance such as silicon oxide is vapor-deposited from an oblique direction to a substrate to form an inorganic film on the substrate, so that liquid crystal molecules are aligned in the direction of vapor deposition. This method is not industrially efficient, although stabilized alignment with a constant tilt angle can be obtained. The second method is a method wherein an organic coating film is formed on the surface of a substrate, and the surface is rubbed with a cloth of e.g. nylon or polyester in a predetermined direction, so that liquid crystal molecules are aligned in the direction of rubbing. By this method, stabilized alignment can be obtained relatively easily, and industrially, this method is mainly employed. As the organic film, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide may, for example, be mentioned. However, from the viewpoint of chemical stability, thermal stability, etc., polyimide is most commonly employed. As a typical example of such polyimide used for a liquid crystal alignment film, one disclosed in JP-A-61-47932 may be mentioned.
The treating method for aligning liquid crystal by rubbing polyimide is an industrially useful method which is simple and excellent in the productivity. However, along with an increasing demand for high performance and high refinement of liquid crystal display devices and accompanying developments of the corresponding new display systems, various problems of the rubbing method have been pointed out. Such new display systems include, for example, a STN (super twisted nematic) system wherein the twist angle of the TN type liquid crystal display is increased, an AM (active matrix) system wherein switching elements are formed on individual electrodes, a FLC (ferroelectric) or AFLC (antiferroelectric) system wherein a ferroelectric liquid crystal or an antiferroelectric liquid crystal is employed. Various problems of the rubbing method have been made apparent such that in the STN system, the contrast is so high that scratches on the surface of the alignment film formed by rubbing tend to display defects, in the AM system, the mechanical force or static electricity by rubbing is likely to destroy the switching elements, or dust formed by rubbing is likely to bring about display defects, and in the FLC or AFLC system, it is difficult to satisfy both quick response and uniform alignment of smectic liquid crystal solely by simple rubbing treatment.
For the purpose of solving these problems, a so-called "rubbingless" alignment method has been studied wherein liquid crystal is aligned without rubbing, and various methods have been proposed. For example, a method wherein photochromic molecules are introduced to the surface of the alignment film, and the molecules at the alignment film surface are aligned by light (JP-A-4-2844), a method wherein a LB film (Langmuir Blodgett film) is employed to align molecular chains constituting the alignment film (S. Kobayashi et al., Jpn. J. Appl. Phys., 27,475 (1988)), a method wherein an alignment film is pressed on a substrate which is preliminarily treated for alignment, to transfer the alignment (JP-A-6-43458), have been studied, but when the industrial productivity is taken into account, none of them can be a substitute for the rubbing method.
Whereas, various methods have been proposed in which periodical irregularities are intentionally formed on the alignment film surface, so that liquid crystal molecules are aligned along such irregularities. The simplest method among them is a method wherein a replica having periodical irregularities is preliminarily prepared, and a thermoplastic film is heat-pressed thereon to copy the irregularities on the film (JP-A-4-172320, JP-A-4-296820, JP-A-4-311926, etc.). By this method, it is certainly possible to efficiently prepare a film having periodical irregularities on the surface, but it has been impossible to obtain practical reliabilities at a level of a polyimide film used for the rubbing method. Whereas, a method has been proposed to form periodical irregularities on a film surface by irradiating a highly reliable polyimide film with a high energy light, such as an electron beam (JP-A-4-97130), .alpha.-rays (JP-A-2-19836), X-rays (JP-A-2-2515) or excimer laser (JP-A-5-53513). However, using such a high energy light source can hardly be regarded as an efficient method for alignment treatment, when the industrial productivity for continuously carrying out alignment treatment uniformly on the entire surface of a large size substrate, is taken into consideration.
On the other hand, as an efficient method for forming periodical irregularities on the surface of a highly reliable polyimide film, a photolithographic method may be mentioned. By virtue of its high insulating property and excellent electrical characteristics, polyimide has been used as an insulating film for semiconductors, and in recent years, a so-called photosensitive polyimide has been developed which has a photo-curing property by the polyimide itself. An attempt has been made to employ this photo-curable polyimide to form periodical irregularities by a photolithographic method. By this method, it has been certainly possible to form irregularities on the polyimide film surface, but the photo-curable polyimide has been initially developed as an insulating film. Accordingly, the property to align liquid crystal has been inadequate, and it has been necessary to coat a buffer layer (JP-A-4-245224), whereby the process has tended to be complex, and in view of the industrial productivity, such a method can not be regarded as an efficient method for alignment treatment which may be a substitute for the rubbing method.
As a new method for alignment treatment recently discovered, a method has been proposed wherein polarized ultraviolet rays, etc. are irradiated to the surface of a polymer film, whereby liquid crystal molecules are aligned without rubbing treatment. For example, the following reports are available.
W. M. Gibbons et al., Nature, 351, 49(1991), Y. Kawanishi et al., Mol. Cryst. Liq. Cryst., 218, 153 (1992), M. Shadt et al., Jpn. J. Appl. Phys. 31, 2155 (1992), and Y. Iimura et al., Jpn. J. Appl. Phys. 32, L93 (1993).
These methods are characterized in that liquid crystal is aligned in a certain predetermined direction by irradiation with polarized light without requiring conventional rubbing treatment. These methods are free from problems characteristic to the rubbing method such as static electricity or scratches on the film surface and have a merit that they are convenient as a production process when industrial production is taken into consideration.
Namely, the method for liquid crystal alignment employing irradiation of polarized light, proposed here, is still in a basic research stage, but is expected to be a method which is prospective as a new treating method for liquid crystal alignment using no rubbing treatment in the feature.
As a film material for liquid crystal alignment used in the previous reports, it has been proposed to employ a polymer compound having photo chemically reactive groups introduced into side chains of the polymer, from the necessity to obtain a photochemical sensitivity to the polarized light. As a typical example, polyvinyl cinnamate may be mentioned. In this case, it is believed that an anisotropy will be formed in the polymer film by dimerization at the side chain portions by light irradiation, whereby liquid crystal is aligned. Further, as another example, it is described that a low molecular weight dichromatic azo dye is dispersed in a polymer material, and polarized light is irradiated to the surface of this film, whereby liquid crystal molecules can be aligned in a predetermined direction. Still further, it is reported that liquid crystal molecules are aligned by irradiating polarized ultraviolet rays, etc., to a certain specific polyimide film. It is believed that in this case, polyimide main chains in the predetermined direction are decomposed by the irradiation, whereby liquid crystal alignment is produced.
With a polymer material having photo chemically reactive groups introduced to side chains of the polymer, as represented by e.g. polyvinyl cinnamate, the thermal stability of alignment is inadequate, and no adequate reliability has been obtained from the practical viewpoint. Further, in this case, the structural moieties which produce alignment of liquid crystal are considered to be side chain portions of the polymer, and such may not be necessarily desirable with a view to obtaining uniform and secure alignment of liquid crystal molecules. Whereas, in the case where a low molecular weight dichromatic dye is dispersed in a polymer, the dye which aligns liquid crystal is of a low molecular weight itself, and from the practical viewpoint, problems remain from the aspect of reliability against heat or light. Further, with the method wherein polarized ultraviolet rays are irradiated to the specific polyimide, as the polyimide itself, the reliability in e.g. heat resistance, is high, but the alignment mechanism is considered to be attributable to decomposition by light, whereby it is likely that in the feature, adequate reliability may not necessarily be obtained from the practical viewpoint.
Namely, in a case where liquid crystal alignment employing this polarized light irradiation is practically applied in the future, it will be required not only to simply align liquid crystal initially, but, from the viewpoint of reliability, to secure more stable alignment. Further, when a practical industrial application is taken into consideration, it is desired to select a polymer structure having also thermally high reliability, and it is also desired to discover a treating agent for alignment using a polymer material having a wider range of selection for the structure. From these viewpoints, polymer materials so far proposed for liquid crystal alignment by irradiation, are not necessarily satisfactory from the aspects of alignment power and its stability, which constitutes a serious problem against practical application of rubbingless alignment by irradiation. The object of the present invention is to provide a treating agent for liquid crystal alignment whereby liquid crystal is aligned by light irradiation to a liquid crystal alignment film without rubbing treatment of the liquid crystal alignment film, and whereby uniform and stable liquid crystal alignment can be secured by a highly heat resistant polymer material.